The NCTM has published a series called "The Addenda Series" for both middleschool and high school. They have interesting stuff!

The Geometer's Sketchpad is a great program and I bet middle schoolstudents would love it. Ask Annie Fetter (annie@forum.swarthmore.edu) formore information. It's published by Key Curriculum press. They also havea "workbook" which is full of interesting explorations and problems.